In recent years there have been many advances in the production of polyolefin homopolymers and copolymers due to the introduction of metallocene catalysts. Metallocene catalysts offer the advantage of generally a higher activity than traditional Ziegler catalysts and are usually described as catalysts which are single site in nature. There have been developed several different families of metallocene complexes. In earlier years catalysts based on bis(cyclopentadienyl) metal complexes were developed, examples of which may be found in EP 129368 or EP 206794. More recently complexes having a single or mono cyclopentadienyl ring have been developed. Such complexes have been referred to as ‘constrained geometry’ complexes and examples of these complexes may be found in EP 416815 or EP 420436. In both of these complexes the metal atom eg. zirconium is in the highest oxidation state.
Other complexes however have been developed in which the metal atom may be in a reduced oxidation state. Examples of both the bis(cyclopentadienyl) and mono(cyclopentadienyl) complexes have been described in WO 96/04290 and WO 95/00526 respectively.
Other known monocyclopentadienyl complexes are those comprising phosphinimine ligands as described in WO 99/40125, WO 00/05237, WO 00/05238 and WO 00/32653. A typical example of such a complex is cyclopentadienyl titanium [tri-(tertiary-butyl) phosphinimine]dichloride.
The above metallocene complexes are utilised for polymerisation in the presence of a cocatalyst or activator. Typically activators are aluminoxanes, in particular methyl aluminoxane or alternatively may be compounds based on boron compounds. Examples of the latter are borates such as trialkyl-substituted ammonium tetraphenyl- or tetrafluorophenyl-borates or triarylboranes such as tris(pentafluorophenyl) borane. Catalyst systems incorporating borate activators are described in EP 561479, EP 418044 and EP 551277.
The above metallocene complexes may be used for the polymerisation of olefins in solution, slurry or gas phase. When used in the slurry or gas phase the metallocene complex and/or the activator are suitably supported. Typical supports include inorganic oxides eg. silica or polymeric supports may alternatively be used.
Examples of the preparation of supported metallocene catalysts for the polymerisation of olefins may be found in WO 94/26793, WO 95/07939, WO 96/00245, WO 96/04318, WO 97/02297 and EP 642536.
WO 98/27119 describes supported catalyst components comprising ionic compounds comprising a cation and an anion in which the anion contains at least one substituent comprising a moiety having an active hydrogen. In this disclosure supported metallocene catalysts are exemplified in which the catalyst is prepared by treating the aforementioned ionic compound with a trialkylaluminium compound followed by subsequent treatment with the support and the metallocene.
Metallocene catalysts have also been referred to as “single-site” catalysts and have been used to prepare polyethylenes having a narrow molecular weight distribution. Linear low density polyethylenes (LLDPE's), prepared from the copolymerisation of ethylene and higher α-olefins in the presence of such “single-site” catalysts typically exhibit a uniform composition distribution wherein the comonomer is uniformly distributed within the polymer chains. The combination of narrow molecular weight distribution and uniform composition distribution distinguishes these polymers from “conventional” LLDPE which is prepared from a Ziegler-Natta catalyst or a chromium catalyst.
The conventional LLDPE products have a broad molecular weight distribution and a broad composition distribution and these properties are seen in the resultant physical properties of the polymers. LLDPE's prepared from a single site catalyst have improved dart impact strength and optical properties compared with conventional LLDPE's. However conventional LLDPE's are easier to process in mixing and extrusion equipment.
It would therefore be highly desirable to prepare LLDPE's which possess the improved physical properties provided by the single site catalysts as well as the improved processability of the polymers prepared by conventional catalysts.
Mixed catalysts have been used to try to achieve this desired balance of properties. For example EP 128045 describes the use of two different metallocenes in a single reactor. The metallocenes are bis(cyclopentadienyl) metal complexes activated by aluminoxanes. EP 232595 describes the use of a supported catalyst prepared from a metallocene catalyst and a Ziegler-Natta catalyst. Again the metallocene catalysts are typically bis(cyclopentadienyl) metal complexes activated by aluminoxanes.
The use of such mixed catalyst systems is often associated with operability problems for example the use of two different metallocenes on a single support as described in the aforementioned EP 232595 may lead to difficulties with process control.
WO 01/05849 describes mixed polymerisation catalyst systems comprising at least two different phosphinimine catalysts supported on a single support which may be used to prepare LLDPE's having a broadened molecular weight distribution. The phosphinimine catalysts typically comprise monocyclopentadienyl ligands as described in WO 99/40125, WO 00/05237, WO 00/05238 and WO 00/32653. These “mixed” monocyclopentadienyl catalyst systems are activated by aluminoxanes for example methyl aluminoxane (MAO) or ionic activators for example borates such as N,N-dimethylanilinium tetrakispentafluorophenyl borate.
WO 97/44371 describes copolymers having a reverse comonomer distribution prepared in a single reactor by use of polymerisation catalyst system comprising a single metallocene complex. By reverse comonomer distribution is meant a copolymer having a comonomer content that is higher in the higher molecular weight fraction.